Binder admixture, kaolin product and their manufacture

ABSTRACT

A binder mix containing a hydraulic binding agent and an admixture mixed therewith, a process for preparing a hydraulically hardened binder mass, a calcinated kaolin product and a process for preparing the same. A paste-like composition is formed from the hydraulically hardening binding agent, a pozzolanically reacting admixture and water, which, when so desired, contains stone aggregate or similar filler, the paste-like composition is worked and the worked composition is allowed to harden to form the binder mass. The admixture comprises spherical, porous metakaolin agglomerates, the size of which is 2-200 microns and which have an open pore structure. The invention can be used to improve the manufacturing methods of cast concrete products and shorten the manufacturing times, and to provide concrete with better mechanical and chemical properties, as well as improve the fire and frost resistance of concrete.

Applicant hereby claims foreign priority benefits under 35 U.S.C. § 119of PCT Patent Application No. PCT/FI02/00853 filed Nov. 1, 2002; FinlandPatent Application No. 20012116 filed Nov. 1, 2001 and Finland PatentApplication No. 20021445 filed Aug. 5, 2002, the disclosures of whichare herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to new kaolin products and theirpreparation and use.

The invention relates to a binder mix in particular, which typicallycontains a hydraulically hardening binder and a pozzolanically reactingadmixture.

The invention further relates to a process for manufacturing calcinatedkaolin and a process for manufacturing a hardened binder mass.

2. Background Information

Metakaolin can be used as an additive of cement. Accordingly, the U.S.Pat. No. 6,027,561 describes a composition that contains cement and ahighly active pozzolan that contains metakaolin. This is manufactured bymeans of treating kaolin thermally, elutriating it in water and dryingthe slurry by spray drying, whereby small agglomerated beads with adiameter of at least 10 microns are formed. They are formed fromparticles, the size of which (d50) is 5 microns or less. A knowndispersing agent can be added into the metakaolin. As fine-grainedpozzolan generally requires a greater amount of water, the purpose ofthe known solution is to provide a product, which has no negativeeffects on the fluidity or the water demand of concrete. The beadsconsist of metakaolin microparticles (70-100%) and dispersing agents andother additives (0-30%). In this way, the fluidity of cement can beimproved compared to a case where fine-grained pozzolan is used.

Other patents, which use metakaolin as additives of cement include U.S.Pat. Nos. 5,976,241, 5,958,131, 5,626,665, 5,122,191 and 5,788,762, thelatter including a versatile overview of the use of metakaolin and itsadvantages as an additive of cement. A common feature for all thesolutions is the use of metakaolin as such, whereby its pozzolanicproperties are utilized.

Microspheres consisting of various materials have been used as additivesof cement. An example of such an application is the solution disclosedin US patent application 20010047741, wherein a porous substance can be,for example, volcanic ash or hollow ceramic microspheres or combinationsthereof. The purpose behind the use of these microspheres is to obtainthe advantage of an essentially lighter-weight substance combination.The density of the porous additive is 0.5-1.2 g/cm³, preferably 0.9-1.1g/cm³. The size of the porous particles is 20-120 microns (micrometres).

In some cases, the spheres that are used to lighten concrete have closedsurfaces, and efforts have been made to actually minimize the amount ofwater absorbed by them. On the other hand, large light expanded clayaggregates, which have also been used to lighten concrete and to alsobring water to the concrete matrix, are so large that their relativelylow strength constitutes the weakest ling in the concrete matrix.

Patent literature has also dealt with the adjustment of the amount ofwater during the hardening reaction of cement. Accordingly, the U.S.Pat. No. 5,948,157 discloses an admixture, the surface of which istreated so that it has a transient hydrophobic property, whereby it willnot deliver water, when the admixture is added into the mix, but willlater on change so that the travel of water during the hardeningreaction of the mixture is possible. The additive can either react withthe mixture during hardening or by means of intermediary reactionproducts or by physically binding itself to the mix. The additive can beany substance, the surface properties of which can be changedtransiently. In a preferred application, the additive is silicate, whichby nature is slightly hydrophilic. The silicate can be formed from areactive material. It is preferably formed from a pozzolanic andhydraulic material or the mixtures thereof. The treating agent can be anorganic oxide that has at least three carbon atoms. In can also be asurface-active agent that includes a hydrophobic component, which has anorganic oxide with 3 carbon atoms. The treating agent covers at leastpart of the surface of the additive and gives it a transient hydrophobicproperty, but does not react with it. The purpose is that the effect ofthe treating agent is valid only for the time of treating the mixtureand, after this, its effect starts to decrease, whereby the additive canparticipate in reactions with the mixture.

U.S. Pat. No. 4,095,995 deals with the manufacture of light concrete. Inorder to avoid the entry of water into the porous aggregate, whenmanufacturing light concrete, an additive is brought onto the surfacesof the aggregates, forming, together with the cement slurry, a gel-likeprotective layer, which only has a limited permeability to water. Whencoating the aggregates, a dry additive is used, which contributes to thecreation of a bonding between the cement and the aggregates. The saidadditive is a polyethylene oxide with a high molecular weight.

Adding water by means of the particles in the concrete matrix during thehardening reaction of concrete, which can be used to decrease autogenouscontraction, is dealt with in the application publication WO 0102317,wherein the said particles are of organic polymer, ‘hydrogel’.

The manufacture and use of round inorganic granules for dyeing concreteis considered, among others, in the U.S. Pat. No. 5,882,395.

Although the number of patents related to the admixtures of concrete andthe control of hardening is fairly extensive, as a whole, the solutionsmentioned above generally only deal with some single phenomena relatedto the properties of concrete, and they have neither been capable ofconsiderably improving the manufacturing processes of cast concreteproducts, nor shortening the manufacturing times, nor providing concretewith better mechanical and chemical properties.

SUMMARY OF THE INVENTION

The purpose of the present invention is to eliminate the disadvantagesrelated to known technology and to provide a new solution for improvingthe mechanical and chemical properties of mixes containing cement andcorresponding hydraulic binders. Another purpose of the invention is toalso improve the properties of hardened binder masses (such as concrete)containing a filling agent (such as stone aggregate), their mechanicalproperties and the fire and frost resistance in particular.

Generally, the purpose of the invention is to provide a new kind of akaolin particle product, which is based on metakaolin consisting ofagglomerates of metakaolin.

The invention is based on the idea that metakaolin agglomerate is formedfirst, and then calcinated to various degrees of calcination, wherebythe density of the surface part after calcination is lower than that ofthe inner part. Calcinated agglomerate is 2-100 micrometres in size,preferably about 5-40 micrometres. The calcination can be brought to thedegree of metakaolin, whereby the amount of crystal water of theproduct's outer surface is typically about 0-8% by weight and that ofthe inner part 2-14% by weight. The calcination can also be continued sothat calcinated kaolin having an exothermic crystal structure isprovided. In the structure, the pore structure can open all the way tothe surface. The agglomerates according to the invention have a goodmechanical strength. Typically, the agglomerate of calcinated kaolinalso has good optical properties.

Products according to the invention can be manufactured, among others,by means of a process, wherein slurry with a high dry content (over 50%of the weight of the slurry, preferably about 60-80%, typically about70%) is used, when forming the agglomerate. For making a calcinatedproduct, the agglomerate is preferably calcinated at a fluid state bymainly using heat that is transformed as radiation. The agglomerates arepreferably classified using an ionic wind method that does not induce agreat deal of mechanical stress.

The process according to the invention for producing agglomeratescombines the spray gas phase method and radiation drying. Metakaolin,which is provided by fluid radiation, is post-calcinated into a crystalform, for example, by means of radiation heat transfer.

According to the invention, an admixture of concrete is provided, forexample. Consequently, according to the invention, an admixturecomprising at least spherical, porous agglomerate that reactspozzolanically on its surface is added into a binder admixturecontaining a hydraulically hardening binder. The admixture can be usedto replace a part, e.g., about 5-35% by weight, preferably about 10-25%by weight of the binding agent. Water is stored reversibly in the porestructure of such spherical, porous agglomerates after processing thebinder mass (the cement mass), whereby the water stored in the porestructure is returned in the subsequent hardening phases of the cementand admixture paste.

The binder admixture used in the invention is characterized in havingbeen provided, at the processing stage, with a capability to absorb, inreal time, and store such a water surplus, which in the treating phaseof concrete advances mixing, casting and dampproofing. This watersurplus, which is evenly stored throughout the matrix, can be utilizedlater on, when it is needed to decrease autogenous shrinkage and theresulting micro cracking.

Porous, mechanically strong mineral agglomerates work as the storage forwater. The material of these agglomerates is preferably selected from agroup of substances, which, when calcinated, provides a pozzolanicproperty at least on the surface of the agglomerate. Typically, such amineral substance belongs to clay minerals and is preferably kaolin,from which metakaolin is developed when calcinated. The known propertiesof metakaolin, which improve the properties of cements of the Portlandtype, are thus combined with a property that is considerable for thehardening reaction of concrete described above, which cannot be providedwhen using metakaolin in a pure powder form.

FI patent application 20012116 describes in detail a process formanufacturing porous particles, which can be used in the presentinvention. They are characterized in comprising metakaolin agglomerates,the size of which is 2-100 micrometres and which have an open porestructure, whereby the density of the surface part of the metakaolinagglomerates is lower than that of the inner part, and their porestructure in the surface part and the inner part is similar.

More specifically, the binder admixture according to the invention ismainly characterized in that, which is presented in the characterizingpart of claim 1.

The process according to the invention for manufacturing a hydraulicallyhardened binder mass is characterized in that, which is presented in thecharacterizing part of claim 12.

The calcinated kaolin according to the invention is characterized inthat, which is presented in the characterizing part of claim 18, and theprocess for manufacturing the same is characterized in that, which ispresented in the characterizing part of claim 19.

The invention provides considerable advantages. Accordingly, itconsiderably improves the manufacturing methods of cast concreteproducts and shortens the production times, as well as provides concretewith better mechanical and chemical properties and improves the fire andfrost resistance of concrete.

By means of the invention, part, preferably at least 5% by weight,preferably as much as 35% by weight, of a conventional hydraulic bindingagent, such as cement, e.g., Portland cement, and/or furnace slag can bereplaced, and, nonetheless, the binder admixture can be provided withgood or even improved properties. The mixture is easy to work; itstiffens quickly after processing and gives the concrete a good finalstrength.

As stated above, various kaolin products can be provided according tothe calcination conditions. A product having an exothermic crystalstructure is well suited to paper filling and coating material of paper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the effect of metakaolin on theviscosity of cement paste, whereby viscosities measured of variousbinder masses (without the stone aggregate) by the Brookfieldviscosimetre are given as the function of solids volume (Example 1);

FIGS. 2A-D show schematically, how the MKAs between cement particleseven out the distribution of hydration products;

FIG. 3 is a graphical representation of the effect of the uniformity ofgel distribution on the compressive strength of cement;

FIG. 4 shows an electron microscope image of a typical microcrack;

FIG. 5 shows schematically the different phases of the drying ofconcrete and those of the resulting shrinkage;

FIG. 6 shows schematically the structure of the bonding between hydratedcement and stone aggregate;

FIG. 7 shows schematically the magnitudes of the particles and the poresin the hydrated cement paste;

FIG. 8 is a graphical representation of the effect of the gel-spaceratio on the strength;

FIG. 9 is a graphical representation of the absorption of water ofmetakaolin agglomerates as the function of time,

FIG. 10 shows the process flowchart of the first unit of the equipmentaccording to the invention,

FIG. 11 shows the process flowchart of the drier unit used in theinvention, and

FIGS. 12 a and 12 b show the process flowcharts of the calcination unitsused in the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the invention is described in detail, particularlyusing the manufacture of the product suitable for the admixture ofconcrete and its use as examples. First, however, a few comments onkaolin and its calcination.

Calcination of Kaolin

The raw material in the process according to the invention are kaolin(natural kaolin) and possible admixtures, such as calcium carbonate orsilica, and the proportion of binder is typically very low (less than1%). Instead of granules, the process forms agglomerates. In granules,the film formed by the binding agent defines the size of the granule anda larger inner pore structure. In agglomerates, the pore structure is aconsequence of particles locking to each other, enabling the manufactureof agglomerate with a strong design so that only the surface layer issintered, whereby bonding necks are formed between individual particles.

It is typical of the agglomerate that

-   1. The shape is indefinable, showing a tendency towards spherical,    however-   2. The pore structure extends in an essentially homogeneous form    throughout the agglomerate-   3. The amount of binding agent is minor (<2%) or nonexistent;    therefore, the film formation in connection with drying has no    effect on the structure-   4. Charges of opposite signs acting on the surfaces and the edges of    the platy particles have an effect on the pore structure of the    kaolin agglomerate-   5. The agglomerate strength is good because of the bonding forces    induced by the charges-   6. The structure is porous, because it has a low degree of    organization 7. The agglomerate is developed, when the dry content    of its raw material, the slurry, is high, typically about 70%,    whereby the structure of the drop created in spray drying is locked    before any essential inner organization is allowed to take place-   8. The size of the agglomerate is very much dependent on the drop    size. Capillary forces are the only ones that have a decreasing    effect on its final size.

Calcinated kaolin is a common term for kaolins, the crystal structure ofwhich is modified by thermal treatment. The temperature and the durationof treatment have an effect on the final crystal form. When theprocessing temperature is less than 1000° C. (typically 500 . . . 700°C.), an essential part of the water (c. 14%) contained by it and boundto its crystal structure exits the crystal structure of the kaolin. Inthat case, a mainly amorphous crystal structure is formed. Generally,the time needed is less than 5 seconds.

As the thermal treatment is continued, the temperature increased and theprocessing time prolonged, the amorphous structure again begins tocrystallize, causing a slight exothermic phenomenon. The crystalstructure thus created deviates from the original crystal structure ofkaolin and from the metakaolin, which was produced as an intermediateproduct. Generally, the product is called calcinated kaolin. The Ticompounds that frequently occur as impurities amongst the kaolin alsochange their crystal form and turn white, increasing the brightness ofthe product by about two ISO units of brightness, which is significantfor the purposes of the commercial product. This change takes place atabout 1000 . . . 1200° C., particularly at about 1100 . . . 1200° C. ata retention time of about one hour. When the slate-like crystal scalesopen, the surface of the product is rough.

Metakaolin agglomerate has an agglomerate structure and it is obtained,when the kaolin agglomerate has been treated thermally at a temperatureof 600-1000° C., the processing time being 0.1-5 sec. In that case, thecrystal water leaves the agglomerate and obtains pozzolanic properties.In some products, it is preferable to use metakaolin agglomerate, thesurface of which is metakaolin and the inner part kaolin (or only a veryfew of its inner parts have lost their crystal water). 4. When acidic Cacarbonate, Ca(HCO₃)₂, is used, Ca oxide, CaO, is obtained as a sinteringauxiliary agent for kaolin surfaces that have no crystal water, the Caoxide combining with water and forming Ca hydroxide, Ca(OH)₂, andreacting with the metakaolin surface. 5. Other suitable substances, suchas silica sol, Na silicate (e.g., soluble glass) or nepheline cyanidecan also be used as binding agents.

To manufacture kaolin agglomerate that has an exothermic crystalstructure, the kaolin agglomerate is calcinated into metakaolin in afirst phase, the radiation temperature being about 1200° C., andtypically about 2% of a sintering auxiliary is used, whereby the porestructure of the surface mainly closes. The metakaolin agglomerates aretransferred into a calcination furnace, wherein the change in crystalstructure, which requires time (about one hour), takes place at atemperature of about 1200° C.

When the product is calcinated agglomerate, it is generallycharacterized in that:

-   -   1. The degree of calcination of the agglomerate corresponds to        the crystal structure created after the exothermic reaction    -   2. The pore structure of the surface part is closed    -   3. The density of the agglomerate in the cross section is        constant    -   4. The agglomerate has good optical properties.

The use of kaolin as the metakaolin agglomerate according to theinvention and as agglomerate having a crystal structure of theexothermic area diversifies the product application possibilities ofkaolin compared to the ones presently used.

Binder Admixture and a Process for Preparing the Same

A particular object of the invention is a binder admixture and a processfor preparing a hardened binder mass (such as concrete). The binderadmixture consists of an admixture, hereinafter also called the“multifunctional admixture of concrete”, which cooperates withhydraulically hardening binding agents, such as Portland cement, furnaceslag or other commonly used cement-bearing binding agents. The grindingfineness of the cement is not critical; it can be, for example, 50-1000m²/kg, particularly about 100-800 m²/kg, generally, the fineness ofPortland cement is about 350-560 m²/kg and than of furnace slag cementslightly higher (600-750 m²/kg).

The additive consists, for example, of pozzolanic, porous, sphericalagglomerates provided by a special burning method described in detailbelow, which are in the order of cement granules, and which have beenprovided with a capability to absorb, in real time, the water surplusadded in connection with the manufacture of concrete. When theseagglomerates are used, the concrete is easy to work at the beginning,but when the additional water has been absorbed into the agglomerates,the concrete stiffens quickly. The spherical shape of the agglomeratesfurthers the workability of concrete.

In the following, the agglomerates are also referred to as “MKA”(metakaolin agglomerate).

At a later stage, this additional water that has been stored evenlythroughout the matrix will be utilized to minimize the inner drying ofconcrete (autogenous shrinkage) and the resulting microcracking. This isa very important issue for concretes with a low water-cement ratio, suchas about 0.3-0.4, in particular. The invention can be used to increasethe water-cement ratio. Curing concrete surfaces according to presenttechnology, which is carried out by water spraying only, does notprovide the additional water inside the concrete, which is needed tocompensate for autogenous shrinkage.

When the particles are originally of a suitable size and there are somuch of them that their mutual distance from one another (the spacingfactor) corresponds to the requirements for frost resistance accordingto modern standards, the spherical pores, which have been emptied ofwater, improve the frost resistance of concrete in use.

The raw material for the manufacture of the agglomerate used in theinvention is kaolin (natural kaolin) and possible admixtures, such ascalcium carbonate or silica, and a possible binding agent, such ascalcium hydrogen carbonate or silica sol. Typically, the portion ofbinding agent is minor (typically 0.5-2, normally less than 1%). Thebinding agent can be dissolved in water.

Slurry with a high dry content is formed from the source material. Dropsare formed from the slurry and it is dried using the spray dryingmethod. Instead of granules, agglomerates are formed by the process. Thepore structure of the agglomerates is the result of particles locking toeach other, enabling the preparation of agglomerate with a strongstructure so that only the surface layer is sintered, whereby bondingnecks are formed between individual particles.

The size range of the agglomerates is 2-200 micrometres, typically 5-100micrometres. Agglomerates with a size (d50) of 20-100 micrometres arepreferably used in the binder admixture. In the manufacture, the size ofthe drops is in the same order, preferably about 5-50% by volume,typically about 7-20% by volume, preferably about 10-15% larger thanthat of the agglomerates obtained as a product.

The kaolin agglomerates obtained from drying are taken to thermaltreatment. The surface of the agglomerate in particular is treated withheat (calcinated) at a temperature, where the crystal water evaporates.Normally, the thermal treatment is carried out at about 570-1000° C. (atabout 600-1000° C.). When the crystal water exits, the kaolin turns intometakaolin. The crystal water forms about 14 percent of the weight ofthe kaolin. If all the crystal water has not been able to exit after thethermal treatment, the inner part of the agglomerate may at least partlyremain kaolin.

In the manufacturing process, agglomerate is thus formed first andtreated thermally to form metakaolin. When so desired, the thermaltreatment can be continued to prepare a calcinated product.

The agglomerate used in the invention is characterized by an open porestructure, i.e., it has “open pores”, meaning that water is free to movefrom the inner parts of the agglomerate onto its surface. In this sense,the agglomerate according to the invention deviates from the granuleused for paper coating, for example, wherein the surface layer is denseand no water or vapour can freely flow through it. The presentagglomerates have also been defined as to having “a pore structure thatis essentially similar in the surface part and the inner part”. Thisrefers to the open pore structure disclosed above. In practice, thepores can be smaller in the surface layer of the agglomerates than intheir inner parts. However, they are always essentially open in order toallow water to move in and out of the agglomerates.

The Ca carbonate, which can be used as an additive/admixture in themanufacture of the agglomerates, is calcinated into Ca oxide, releasing,at the same time, gaseous carbon dioxide. This fills at least some ofthe agglomerate pores. Alternatively, the agglomerate is formed fromkaolin only, whereby its surface is changed into metakaolin by thermaltreatment, while the crystal structure of the inner part remains kaolin.

The surface tension in the drops tends to organize the agglomerates inan essentially spherical form. The agglomerates according to theinvention are spherical in that their deviation from the shape of a ballis smaller than 30%, preferably smaller than about 20%, most preferablysmaller than about 10%.

As stated above, the agglomerates typically have an open, porousstructure, whereby the pore structure extends essentially homogenouslythroughout the whole agglomerate. Generally, the portion of pores fromthe agglomerate volume is about 20-80% by volume, preferably about30-70% by volume, most preferably about 40-60% by volume. The size ofindividual metakaolin/kaolin particles ranges fairly extensively, butgenerally it is about 0.1-50 micrometres, usually about 0.5-40micrometres.

It should be mentioned that the agglomerate structure is porous, becauseits degree of organization is low. As no binding agent is added into theslurry, or there is not a lot of it (<2%), therefore, the film formationthat takes place in connection with drying does not affect thestructure.

Charges of opposite signs, which act on the surfaces and the edges ofthe platy particles, affect the pore structure of the kaolinagglomerate. The strength of the agglomerate is good because of thebonding forces induced by the charges.

The agglomerate described above is generated in particular, when the drycontent of its raw material, the slurry, is high, typically over 50% byweight, preferably over 60% by weight, usually 60-80% by weight, wherebythe structure of a drop generated in spray drying is locked before anyessential inner organization takes place.

The invention provides a process for preparing a hydraulically hardenedbinder mass. In such a process, a paste-like composition is formed froma hydraulically hardening binding agent, a pozzolanically reactingadmixture and water, containing, if so desired, stone aggregate or asimilar filling agent. The paste-like composition is worked(mixed/vibrated and cast, etc.) by a process known per se so that itbecomes homogenous, and the worked composition is allowed to harden toform the binder mass.

At the initial stage of processing the concrete (e.g., casting), thewater required by the processing begins to penetrate the pore structureof the porous agglomerates in the binder admixture so that the pores inquestion become partly filled.

The need for special plasticizers decreases because of the improvementin the workability, which is caused by the round shape of theagglomerates. It could even be harmful to use them, if they boundthemselves partly to the particle surfaces that should be as reactive aspossible.

As the manufacturing process of the said agglomerates leaves the saidpores filled with gas, which is either air, carbon dioxide or a mixturethereof, the gas must exit the pores so that water can penetrate them.It has been observed that the said microbubbles plasticize cement paste.The portion of carbon dioxide in the discharging gas reacts with freecalcium hydroxide and forms calcium carbonate.

The outermost strong coat of the particles has effectively been madehighly pozzolanic and preferably further coated so as to be alkaline,whereby hardening starts quicker and the final strength is higher.Furthermore, because of the pozzolanic property and the structure of theparticles, they provide, together with the cement, a better matrix asregards the long-term durability of concrete.

Generally at least 50% by weight of the mixture formed from the bindingagent and the admixture consists of a hydraulically hardening bindingagent, such as cement (e.g., Portland cement) or furnace slag. About 35%by weight, at the most, consists of the pozzolanically reactingmetakaolin agglomerate described above. Generally, the average size(d50) of the porous agglomerates is 5-200 micrometres, preferably over30 micrometres, as stated above. The average size (d50) of theagglomerate is most preferably the same or essentially the same as theaverage size (d50) of the particles of the cement-containing bindingagent used in the mixture.

When using agglomerates that have such a size and an amount that, in themixture, their mutual distance from one another (the spacing factor) isless than 200 mlcrons, on the average, the porous agglomerates emptiedfrom water in the hardened mixture are able to improve the frostresistance of concrete. This feature is examined in detail below.

The binder admixture can be hardened from one containing agglomerates toform a binder mass, which has a porous matrix formed from strongagglomerates, the distance between the pores in the matrix being 5-500microns, preferably 20-100 microns. It can especially be hardened toform such a hardened binder mass, the 28d compressive strength of whichis 60-160 MPa, preferably 80-120 MPa.

In the following, the properties of the agglomerates in cement mixes andconcretes are described in detail.

The advantages of using metakaolin in connection with Portland-typecements are well known.

The most important advantages are:

-   -   1. They increase the strength, the durability and the brightness        of concrete.    -   2. They prevent the harmful alkali-silica reaction.    -   3. They decrease the harmful effect of acidic substances on        concrete    -   4. They decrease the formation of lime mildew that often forms        on the surface of concrete.

The strength gain is based on the fact that metakaolin as pozzolandecreases the amount of thus created calcium hydroxide in the hardeningcement mass. Because of its cleavage faces, which form easily, calciumhydroxide is a weak link in the cement matrix. At the same time, itreduces the porosity of the hardening concrete and tightens anddecreases the thickness of the so-called transition zone, whereby thebonding force between the cement and the stone aggregate increases. Ithas been observed that metakaolin reacts with such an amount of calciumhydroxide, which is greater than the added metakaolin, being as much as2-5 times more effective than other pozzolans commonly used, such as flyash. Metakaolin does not slow down the hardening reaction. Generally,the final strength is better than that of concretes without metakaolin.

The growth of the durability of concrete is manifested in various ways.The use of metakaolin improves, among others, resistance to sulphatesand chlorides and reduces any problems caused by freezing. Resistance toacids is also improved. Metakaolin tightens the concrete structure,whereby it decreases the amount of harmful alkali metals and chloridesin the water, which is entrapped inside the pores, and decreases thepenetration of chlorides into the concrete matrix. It has been observedthat, from the point of view of water absorption, pores of 0.05-10microns are significant. As metakaolin reduces the number of thesepores, both the absorption volume and rate of water decrease. Althoughmetakaolin increases the acidity of the pore water; nonetheless, italways remains over pH 12.5, which is considered a recommendable valuefrom the point of view of steel corrosion.

Metakaolin has not been discovered to have any harmful effects on theuse of plasticizers and accelerants.

The effect of metakaolin on the so-called alkali-silica reaction isbased on the fact that it reacts with the calcium hydroxide thuscreated, and thus prevents any disadvantages caused by the gel createdby the reaction between the calcium hydroxide and active silica. The gelexpands and causes quick disintegration of the structure in asaltbearing environment in particular.

The use of metakaolin in the prevention of lime mildew is also based onbinding free calcium hydroxide. Perfect prevention of mildew requiresthe use of larger dosages; therefore, it cannot be removed completely bymeans of the dosages commonly used.

The use of agglomerates provides considerable advantages, among others,an improvement in workability, growth of the early strength, and a highfinal strength.

The shape of the agglomerate according to the invention is spherical.The volume taken by them from the volume of the binder mass ispreferably about 50%. In that case, in an optimal mixture, the cementparticles and the agglomerates of essentially the same size are fullymixed and in the immediate vicinity of each other. The sphericalagglomerates improve the rheological properties, i.e., the fluency andworkability of the mixture without using separate plasticizers.

FIG. 1 shows viscosities measured of different binder masses (withoutstone aggregate) by means of the Brookfield viscosimetre as a functionof volume of the solid matter (Example 1). The figures clearly show,that when MKA is used as part of the binder admixture, its degree ofviscosity is clearly lower. In the situation presented, ⅓ of the cementwas replaced by an addition of MKA having almost the correspondingvolume (⅙). The smaller weight fraction of MKA can be explained by itslightness (about 1.1 kg/dm3) compared with cement (the density is 3.1kg/dm3). As it is significant for the reactions and interactions betweenthe particles, the proportion of volume parts is examined herein inparticular.

The general compression density of Portland cement (the volume part ofthe solids from the binder mass) is about 40%. According to measurement,the viscosity is then 1120 mPas. The corresponding compression densityof the mixture, into which MKA was added, on the same degree ofviscosity is higher, i.e., 47%.

If the object is to provide a structure according to a cubical packingof spherical particles of a theoretically uniform size, thecorresponding compression density is then about 52%. This can beprovided by the addition of MKA, whereby the viscosity is about 2500mPas, which can still be worked but, however, not very well. Forconventional Portland cement, this compression density cannot beachieved at all within the limits of workability without the addition ofother possible chemicals.

To achieve sufficient workability, concrete needs more water than thehydration of the cement that works as the binding agent. As concreteneeds a certain amount of cement paste, the amount of water cannot beinfluenced by means of reducing the amount of cement. Generally usedfine-grained admixtures, such as silica, do not decrease the amount ofwater but only divide it into an even more fine-grained form.

Neither does MKA reduce the amount of water more than its sphericalshape and pore structure allow. It is essential that the excess watercan be stored in a reversible manner in the strong porous structure ofthe agglomerate.

Between cement particles, there are agglomerates that are suitablyproportioned, whereby they work as ‘bridges’, when the cement particlesare hydrated, thus reducing the length of travel of the hydrates (FIGS.2 a-d). Because of the lower density of MKA, a proportioning between theparticles can be provided by means of a moderate addition of MKA,wherein MKA is in the immediate vicinity of the cement particles.

After the water required by processing has been absorbed into the porevolume of the MKAs, the cement particles and MKAs approach each otherand the cement paste stiffens quickly, which is significant for theeconomy of construction engineering. In that case, shuttering can bedetached earlier than at present.

Example 2 describes the ability of MKA to absorb water into its porestructure. This property can be adjusted in connection with themanufacture of MKA, for example, by changing the conditions ofcalcination and, thus, the porosity of the agglomerate surface. Thephenomenon can also be influenced by providing calcium oxide onto theagglomerate surfaces in connection with the manufacture, which calciumoxide reacts with the carbon dioxide entrapped in the agglomerate afterobtaining some moisture.

Efforts have been made to influence the rate of the strength gain ofcement and the 28d strength gain by increasing the grinding fineness ofthe cement, i.e., releasing the entire cement reserve within a shorterperiod of time. However, increasing the grinding fineness has thedisadvantage that, after the cement has fully reacted at the initialstage of its life span, no inert cement and strength potential are leftto be used for subsequent damage of the concrete matrix, but the damagecan advance easier than if coarser and slowerreacting cement had beenused.

The said problem can be solved using metakaolin as the admixture of thecement. Coarser cement can be used and, nonetheless, a high earlystrength can be achieved, and the inert portion of the cement remainsand increases the long-term durability of the concrete. This is based onthe fact that the space of cement hydrates that is to be filled has beendecreased and the length of their travel shortened.

A theory presented by Verbec-Helmut [1] deals with the effect of a densehydrate layer on the strength and the low solubility of the hydrationproducts of cement and the drift of diffusion products and the resultingnon-uniform distribution. This theory and the calculation modelpresented by Neville [2] have been used to illustrate the invention inthe following. No stand is taken here regarding the validity or theaccuracy of the appended calculation model, but it has been discoveredto correlate with obtained measurements and to facilitate thecalculative examination of the process.

The effect of the dense hydrate layer on the strength can be explainedby means of the ratio between the volume of the hardened cement gel andthose of the hydrated cement and capillary pores. The strength of thecement increases, when the ratio between the gel volume and thecorresponding available space increases. When the ratio of the volume ofhardened cement gel to the sum of the volumes of cement gel and freewater is about 2.06-fold, the ratio can be expressed by means of thefollowing formula:

$\begin{matrix}{{X = \frac{2.06\mspace{14mu} v_{c}c\;\alpha}{{v_{c}c\;\alpha} + W}},{wherein}} & (1)\end{matrix}$

-   v_(c)=specific volume of cement, dm³/kg-   c=weight of cement, kg-   α=degree of hydration-   W=water volume, dm³

Example 3 shows an example of the use of the formula in evaluating thecement strength.

The non-uniform distribution of the hydration products of cement isclearly demonstrated, when heat is used to accelerate the hydration. Thelocation of agglomerates between the cement particles and the increasein solids proportion shorten the length of travel of the hydrationproducts, while the binder admixture hardens and, at the same time,decreases the non-uniform hydrate distribution. The so-called failure offinal strength that occurs in connection with the heat treatment ofconcrete is mainly the consequence of a non-uniform hydratedistribution.

FIG. 3 shows the effect of the uniformity of gel distribution on thecompressive strength of cement. When hardened cement has a certaingel-space ratio and the distribution of hydration products is uniform,better strength is achieved than if the gel-space ratio of hardenedcement is approximately the same but the distribution of hydrationproducts is non-uniform. In that case, namely, those parts of hardenedcement, wherein the gelspace ratio is lower than average, limit theachievable strength.

Low solubility and diffusivity of the hydration products of cementcomplicate the capability of the products to substantially move from thesurface of a hydrating cement granule during quick hydration.

Modern concreting technology requires the acceleration of concretestiffening by means of heat. This results in the so-called finalstrength failure, i.e., a nonuniform distribution of hydration products.The schematic presentation of FIG. 2 shows how the MKAs between cementparticles even out the distribution of hydration products.

The use of MKAs results in only a minor failure of the final strengthcaused by the accelerated stiffening of concrete.

One important advantage, which can be achieved by means of theinvention, is the prevention of the formation of microcracks inconcrete.

The most common reasons for the microcracks in hydrated cement orconcrete are as follows:

-   -   the amount of mixing water is greater than that of hydration        water, whereby pores remain    -   certain intermediary products of hydration have larger volumes        than the final products, so because of the fluctuation in        volume, the matrix breaks or retains tensions    -   at the gelling stage, the solid gel expands and binds a larger        amount of water than the final crystallized product    -   internal generation of heat provides thermal strain    -   the different components of concrete and/or plaster have        different elastic modulus, whereby strain is not evenly        distributed    -   the binding agent has not been evenly distributed    -   the finer particles of binding agent have formed agglomerates,        between which not even water can penetrate in normal mixing of        concrete or plaster    -   no bonding forces are generated on the surface of the filler or        the stone aggregate to keep the material together on the        interface between the binding agent and the filler.

FIG. 4 shows a typical microcrack.

As far as the size and dosage volume are concerned, MKA is close to thehydratable Portland cement particle, whereby it works as a certain kindof a pressure accumulator decreasing the pressure and tension peaks.

MKA can primarily be used to affect the so-called autogenous shrinkage.When the value of the water-cement ratio is less than 0.42, the chemicalshrinkage of cement starts to dominate and become harmful, when thetotal amount of water is not enough for complete hydration. The volumeshrinks by 8% when the cement reacts. In that case, a partial vacuum ofas much as 100 kPa develops inside the matrix, and, if no water is thenavailable, microcracking occurs. FIG. 5 shows schematically thedifferent phases of concrete drying and the resulting shrinkage.

Under the effect of partial vacuum and heat, MKA delivers some of thewater it has stored in its pore structure, concentrating the risk ofmicrocracking on its own strong structure.

The need for water comes in impulses and MKA balances it. The amount ofwater stored in MKA roughly corresponds to the difference between theamount of water required by the workability of the paste formed from thebinding agent and the admixture and the smaller amount of water neededby the hydration reactions. In other words, the water, which isredundant for hydration, but which, however, is necessary for processingthe mass, is temporarily transferred to a storage formed by theagglomerates, from where it is released during the hardening of themass, the final hardening in particular, which can be used to preventthe formation of microcracks, among others (see below). Furthermore, thepores absorb gas, such as air, from the concrete mass after beingreleased from water, improving the strength of concrete.

The stone aggregate used in concrete always contains microcracks. Forexample, granite, which mainly consists of the following minerals:orthoclase, biotite and quartz. Biotite by nature is slate-like andcracks easily. Microcracks also easily occur on the interfaces ofvarious minerals. When the stone aggregate is surrounded by a strongcement matrix, the growth of microcracks is prevented and, at the sametime, the cement matrix and the stone aggregate start working as anintegral product, whereby the original difference in their elasticmodulus is no longer significant. The density of the transition zonebetween the cement and the stone aggregate is about half of that ofPortland cement (FIG. 6). The character of the said transition zonechanges in a cement matrix that is blended with MKA. Metakaolin reactswith calcium hydroxide, the agglomerate provides the matrix with astrong spherical structure, and it binds the silicate zone with‘bridges’, the distance of which from each other is about 20-40 microns.In addition, MKA removes excess water from the vicinity of the stonesurface, which is one of the reasons for the sparse structure of thetransition zone.

When examining hardened concrete by means of a microscope, it can beobserved that a fairly big part, as much as half of the surfaces of thestone aggregate, can be improperly attached or even off the cementmatrix. There are a lot of reasons for this problem; among others, firmengagement of air gases to the outermost surface layer in addition tomoisture and stone dust at the moment of crushing, the shape of thestone aggregate, the mixer efficiency has not matched to the stiffnessof the concrete, the gas phase on the stone surface has not been able todetach, the cement particles do nothing but pointedly touch the stonesurface, and there is relatively more water on the surface, whereby alocally high water-cement ratio, dust concentration of the stoneaggregate, a strong local shrinkage of the cement matrix are provided,the capillary water of the cement accumulates onto the surfaces of thestone aggregate, causing capillary porosity on the surfaces, or air isnot allowed to exit in connection with casting. MKA, for its part,eliminates some of the shrinkage of the hydrated cement and theaccumulation of capillary pores, and helps gases to exit the concretebecause of the low viscosity.

Several facts affect the uneven distribution of the binding agent, theeffect of which cannot be totally eliminated by means of the propertiesof the binding agent only. These include the shape of the stoneaggregate and the dust concentration on its surface, or the use of toolow an energy intensity in mixing. The spherical shape of MKA decreasesthe viscosity and the dilatant nature of the binding agent paste,whereby the same energy intensity of mixing provides a better bindingagent distribution in the concrete. When the water bound to the surfaceof the stone aggregate is absorbed into MKA, it cannot affect thewater-binder ratio. The dust bound to the surface of the stone aggregateobtains more moisture under the effect of the moisture obtained throughMKA, whereby a better contact between the stone aggregate and thebinding agent is provided.

As can be observed from the examples above, MKA decreases the pores andmicrocracks of the binding agent paste. They have the effect ofimproving the joining of the binding agent, the aggregate (the-stoneaggregate) and steels together. The tensile stresses of concrete burdenthe steels, the extension of which in loading exceeds that of concrete.While concrete does not crack after casting, the surface area ofconcrete, which is larger than that of steels, receives so much loadingthat the extension of steels induces no cracking.

A substantial part of Portland cement consists of clusters of coreparticles (5 microns) and smaller (2 microns) particles, which surroundthem. The van der Waals forces between the particles are so weak thateffective mixing can disperse the particles from each other. However,the mixing processs presently used are generally not capable of doingthis. When MKA is used, the cement can be rougher-grained, whereby theamount of fines contained by it and also the problematic agglomeratesthat have not been broken up is smaller.

The porous agglomerate according to the invention improves the fireresistance of concrete by various mechanisms. The agglomerates reducethe number of capillary pores and, thus, also the amount of freecapillary water, because the water is absorbed inside the strongagglomerates. Because of an advantageous volume-surface area ratio, thespherical shape and the sintered structure of the agglomerate providegood pressure tightness.

As part of the cement normally used is replaced with the agglomerate,the amount of free calcium hydroxide, which disintegrates in the heatinduced by fire, decreases. In connection with the hardening of cement,about 25% by weight of calcium hydroxide is generated from the cement.The disintegration reaction of calcium hydroxide that takes place in theheat provides one mole of water per each mole of calcium hydroxide. Asthe water is inside the hardened cement structure, it can, when heatingup, provide a pressure of as much as 1000 bar, if the structurewithstands.

The porous agglomerate reduces the amount of calcium hydroxide in twoways. On the one hand, using pozzolanic binder agglomerate can actuallydecrease the amount of cement by 30%, for example. The amount of calciumhydroxide thus generated decreases in proportion. On the other hand, thepozzolanically reacting agglomerate reacts with free calcium hydroxideso that one mole of metakaolin reacts with at least one calciumhydroxide mole, further decreasing the amount of water generated by theheat, and thus improving the fire resistance of the concrete.

The frost resistance of concrete can be improved, among others, byadding air pores into the cement mass. It has been observed that poresof a size of 50-200 microns, the mutual distance between which is lessthan 200 microns, provide concrete with good frost resistance. FIG. 7shows schematically the magnitudes of the particles and pores in thecement.

When the amount of porous agglomerate according to the invention is 20%by volume of the amount of binder, a pore structure is provided that haspores with a size of 50 microns at an average distance of about 100microns from each other.

In has been observed that the penetration of water into the agglomeratepores can be accelerated, with certain limitations, by means ofmechanical methods, among others, which are generally used indampproofing concrete, such as vibration and pressurization.

As the reaction of cement with water, i.e., hydration, advances, morewater is obviously needed; especially when the water-cement ratio usedis low. The surface of the concrete that is hardening by means ofconventional technology is moistened. However, as now only the surfaceparts obtain the necessary additional water, this is not the mostadvantageous practice for the optimal hardening of the structure. It ismore preferable, if the extra water needed by the hardening reaction isnear each cement particle in the entire structure. The hardening ofcement is an exothermic event, which results in an increased temperatureand an increase in the pressure of the residual gas in the agglomeratepores, contributing, at this stage, to the transfer of water outside theagglomerate. The under-pressure and diffusion, which are generated inconnection with the hardening reactions, also have an effect on thistransfer. The volume left by exiting water is partly replaced with theair in the concrete, water vapour and the expansion of the gas stillremaining in the pores. As the gas volume contained by the agglomeratesprovides a sufficient protection against frost, the use of extra air(typically 6% of the cement volume) utilized according to conventionaltechnology for improving the frost resistance is no longer necessary.

The appended drawings 10 to 12 illustrate in detail the process and theequipment according to the invention.

FIG. 10 shows a device for increasing the dry content of kaolin slurryto an area that is advantageous for forming kaolin agglomerates.

Reference number 1 refers to kaolin (natural kaolin) water slurryobtained from a factory, its dry content generally being less than 10%by weight. The slurry is centrifuged, whereby the solids content can beincreased to about 40% by weight. Reference number 2 refers to a waterseparator based on the use of an electric field. Typically, the fieldvoltage is about 1-20 V, preferably about 4-10 V, more preferably about6 V. In the device, the solids content is increased from 40% by weightto 70% by weight. Reference number 3 represents a rotor diffuser, whichis used for the elutriation of kaolin cakes that have a high drycontent. The thus obtained mass is pumped forward by means of ahigh-pressure pump at a pressure of 200-500 bar and into the dryingdevice shown in FIG. 4. A pre-feed pump (which has no reference numberof its own) and the high-pressure pump are capable of treating slurrywith a rigidity of 1000 cP.

The assembly shown in FIG. 10 is continuous. It replaces, for example,the pressure filtration process that is commonly used.

The tower drier according to FIG. 11 prepares the kaolin agglomerateparticles. The equipment comprises a cylindrical container space, intowhich kaolin slurry is fed 5 from the hoisting device of solid matter.The slurry is fed under pressure into nozzles 6, from which the slurryis sprayed into the container space, where the drops dry and formagglomerates. Feeding the high-pressure slurry is carried out in a dropsize of 2-50 micrometres and at a solids content of 70%, whereby thesolid matter is locked into agglomerates in a few milliseconds. Dryingair guides 7 are used for drying, through which guides air is conductedinto the container space, fluid air 11 at 400° C. in particular. Thewater-carrying agglomerates fall onto a wire 9 and the fluid gas isseparated 12 and conducted to a jet condenser. The temperature-of thefluid gas is over 100° C. and it is taken forward to the jet condenser.The agglomerates are dried with radiating heat 10 and transferred bymeans of a fan 13 to ionic wind classifiers 14 and further through atransfer tube system 15 to intermediate storage silos.

Calcination of kaolin agglomerates is carried out in the calcinationequipment according to FIGS. 12 a and 12 b, respectively. First, theequipment has intermediate storages (silos) 16-18, wherein agglomeratesof three particle sizes, for example, are stored separate from eachother. Typically, in the first silo 16, agglomerates are stored, theaverage size of which is less than 10 micrometres. The size ofagglomerates in the silo 17 is 10-20 micrometres, and in theintermediate storage 18, the size of agglomerates is over 20micrometres.

The agglomerates are transferred from the intermediate storages 16, 17and 18, respectively, by means of a fan 19, to the batcher 20 of acalcination furnace, wherein the agglomerates are also mixed. Theagglomerates are calcinated into metakaolin in a radiation furnace 21.The calcinated agglomerates are cooled to 400° C. by a fan 22 and blowninto an ionic wind classifier 23, from where they continue either tostorage 27 or to an exothermic calcination furnace 24. The calcinatedagglomerates are cooled to a temperature of about 400° C. by means of afan 25, and transferred to storage after an ionic wind classifier 26,and the gas is blown to a tower drier 28 at a temperature of 400° C. Theembodiment in FIG. 5 b differs from the solution in FIG. 5 a in that allthe metakaolin obtained from the first calcination are recalcinated 24,whereby the ionic classifier 23 is omitted from the process.

The process described above includes considerable advantages:

-   1. Combined structure    -   Agglomeration-calcination provides energy savings    -   When a precise calcination temperature is obtained, the        classification into different fractions improves the quality of        the product    -   The range of applications of the product grows-   2. The amount of binding and sintering admixtures can be minimized-   3. The degree of calcination between the surface and the inner part    of the agglomerate can be made different-   4. In the formation of the agglomerate, complementing the gas phase    heating with radiation heating brings about considerable savings in    energy.-   5. When high solids content (70%) is used, the gas phase heating,    the energy economy of which is less advantageous, is restricted to a    minimum-   6. Calcination to the metakaolin phase by means of the fluid    radiation process makes the calcination accurate (the time 0.1-0.5    s)-   7. Calcination to the exothermic crystal structure takes time (about    1 hour) and is carried out for metakaolin agglomerates in a grate    furnace, for example. The metakaolin phase has removed the ‘edges’    from the agglomerates so that they are not sintered to each other.-   8. The ionic wind classification treats the agglomerates gently and,    for its part, saves the binder.-   9. The ionic wind classification also saves energy compared with    cyclones and other mechanical classifiers

Products manufactured by the above-described process are suitable forvarious uses, which are dealt with in detail in the following:

Admixture of Concrete

The admixture of concrete and its use are already described in detailabove. Let us mention that a wide size distribution of agglomerates isgenerated in the tower drier, of which agglomerates those over 20microns are very well suited to admixtures of concrete. Theseagglomerates, which are in the same order as cement particles, settleamong Portland cement particles, acting, among others, as pozzolans anddeliverers of water at the phase of the autogenous reaction of cement,and as exact storage for the various additives of cement.

Larger metakaolin particles with a size of over 50 microns act aspozzolanic fillers and plasticizers of concrete.

Paper Filler

According to the invention, one purpose of paper filling is to make partof the calcinated kaolin stick to the fibres, but the majority of theagglomerates, the composition of which can either be metakaolin orcalcinated kaolin that has undergone an exothermic transition phase,settles between the fibres, providing an even web with advantageousoptical and mechanical properties of paper.

Paper Coating Material

Metakaolin or calcinated kaolin that has undergone the exothermictransition phase can also be used for paper coating.

EXAMPLES

The agglomerate product used in the examples was prepared by the processdescribed in FI patent application 20012116. An MKA fraction was used inthe tests, its agglomerates being 5-100 microns in diameter.

Example 1

The example examines the effect of MKA on the viscosity of binder paste.First, cement paste with different water-binder ratios without stoneaggregate was prepared in the test, using general-purpose cement(Portland cement). The viscosity of the paste was measured by theBrookfield viscosimetre after a fixed time period from the mixing. Theresults are described in FIG. 1. After this, another mix was prepared,wherein ⅓ weight fraction of the cement was replaced with ⅙ weightfraction of MKA. The smaller weight fraction is a consequence of thelower density of MKA (3.15 kg/dm³ vs. 1.0 kg/dm³).

As the curve indicates, the viscosities of the cement paste blended withMKA are on a considerably lower level, or the dry content thatcorresponds to the same viscosity is higher for the paste blended withMKA. Formula (1) can be used to calculate that, at a point correspondingto the dry content of 40%, wherein the viscosity is 1120 mPas and whichis still on an area that can be worked, x=0.59 is obtained, and thecorresponding strength from the formula (8) is 42 MPa. The correspondingwater-binder ratio is 0.48. When a paste blended with MKA is prepared,x=0.70, the corresponding strength being 78 MPa. This means that theearly strength grows by 86%.

A stripping strength of 40 MPa is achieved already at a degree ofhydration of α=0.30, i.e., stripping can take place on a considerablylower hydration level and, consequently, considerably quicker thanbefore.

Example 2

The example examines the absorption of water into MKA. In the graph ofthe formula (9), the portion of the pore volume of the particles is usedas the y-axis, the portion having been filled with water as a functionof time t. 1.0 kg/dm³ is used as the density of the agglomerate. Theestimated pore volume in that case is about 50%.

The graph shows that the space between MKA particles (63%) is filledafter 5 minutes, 50% of the pore volume of the particles was filled in 9minutes, and 90% in 17 minutes.

Example 3

The example demonstrates an advantage provided by the invention toindustrially produced pre-cast floor slabs of buildings, i.e., so-calledhollow-core slabs, which are manufactured by extruder machines.Considerable factors in industrial production are the casting rate, theearly strength, the measurement accuracy of the slab and the soil creep,i.e., subsequent changes on the compression side.

In the case of the example, the highest casting rates of the machines,about 4 m/min, are reached when the amount of slurry to be cast is

Mass, kg Density, kg/dm³ Volume, dm³ % by volume Cement 330 3.15 105 39Water 165 1.00 165 61 (Water-cement ratio = 0.5)The effect of the gel-space ratio on the strength is examined inaccordance with the formula (1)

$X = \frac{2.06\mspace{14mu} v_{c}c\;\alpha}{{v_{c}c\;\alpha} + w}$wherein

-   the constant describing the change in the cement volume during    hydration=2.06-   v_(c)=specific volume of cement, dm³/kg=0.32-   c=weight of cement, kg=330-   α=degree of hydration=0.6-   (The value has been selected in accordance with the present    practice, corresponding to a situation, where stripping ccan take    place)-   w=water volume, dm³=165

Using the above parameters, a value of 0.57 is obtained for the ratio,corresponding to the strength of 40 MPa on the graph according to FIG.(3).

When the final strength has been reached, the degree of hydration is0.8. The value of the ratio is then 0.7, the corresponding strengthbeing 75 MPa.

When MKA admixture is used to replace part of the binder, thecorresponding calculation is as follows:

Mass, kg Density, kg/dm³ Volume, dm³ % by volume Cement 200 3.15 63Solid matter MKA 65 1.0 65 Total 49% Water 133 1.00 133 51%

-   (Water-binder ratio=0.5)    When taking into account that, in the example, the amount of water    absorbed by MKA is half of its weight, a corresponding examination    of the gel-space ratio indicates

${X = \frac{{2.06\mspace{14mu} v_{c}c\;\alpha} + V_{MKA}}{{v_{c}c\;\alpha} + V_{MCA} + w_{1}}},$wherein

-   v_(c)=specific volume of cement, dm³/kg=0.32-   c=weight of cement, kg=200-   α=degree of hydration, %=0.4-   (The value α is selected, which provides the same level of strength    as the conventional cement mix)-   V_(MKA)=volume of metakaolin agglomerates, dm³=65-   w₁=water volume, dm³=147−32=115-   (Total water volume−the water volume absorbed by the agglomerates)

In that case, the ratio obtains a value of 0.57, which corresponds tothe strength of 41 MPa, i.e., the same strength was achieved quicker ata lower degree of hydration (0.4 vs. 0.6 for unmixed cement). If weexamine the development of strength corresponding to the same degree ofhydration (0.6) as that of unmixed cement, a value of 0.66 is obtainedfor the gel ratio, corresponding to the strength of 63 MPa. Accordingly,the strength corresponding to the same degree of hydration has grown by54%, when part of the cement is replaced with MKA. The correspondingcalculation for the final strength (α=0.8) indicates that a value of0.74 is obtained for the gel ratio, corresponding to the strength of 90MPa, i.e., the growth of the final strength thus obtained is 20%. Thisexamination does not try to optimise the properties of the product butit indicates that the economic significance of the invention in themanufacture of the product is considerable.

REFERENCES

-   1. Verbeck, G. J., Helmuth, R. H. Structures and physical properties    of cement paste, Proceedings of the 5^(th) international symposium    on the chemistry of cement, Tokyo, 1968, vol III, pp. 7-12-   2. Neville, A. M., Properties of concrete, Pitman Publishing    Limited, London, 1977, pp. 276-285

1. A mix containing a hydraulically hardening binder and apozzolanically reacting admixture, the pozzolanically reacting admixturecomprising spherical, porous calcined agglomerates, said agglomeratesbeing formed prior to calcination, said calcined agglomerates at leastpartly consisting of metakaolin particles; and whereby the size ofindividual agglomerates is 2-200 microns.
 2. The mix according to claim1, wherein the mix is containing at least 50% by weight of thehydraulically hardening binder.
 3. The mix according to claim 2, whereinthe mix is containing about 35% by weight of the pozzolanically reactingmetakaolin agglomerate at the most.
 4. The mix according to claim 1,wherein the pores of the admixture are filled with gas before processingthe mix, whereby the said gas is carbon dioxide or air or a mixturethereof.
 5. The mix according to claim 1, wherein the size of the porousagglomerate is 5-200 microns.
 6. The mix according to claim 1, whereinthe average size d50) of the porous agglomerate is the same oressentially the same as the average size d50) of the particles of thecement-containing binder used in the mix.
 7. The mix according to claim1, wherein the size and the number of the agglomerates are such that, inthe mix, their mutual distance from one another (the spacing factor) isless than 200 microns, on the average, whereby the porous agglomerates,which were emptied of water in the hardened mix, are capable ofimproving its frost resistance.
 8. The mix according to claim 1, whereinon the surface of the agglomerate, there is calcium oxide which, whenreacting with water, changes into calcium hydroxide and starts to reactpozzolanically with the metakaolin.
 9. The mix according to claim 1,wherein the mix can be hardened to form a binder mass, which has aporous matrix formed by strong agglomerates, in which matrix the gapbetween the pores is 5-500 microns.
 10. The mix according to claim 9,wherein the mix can be hardened to form a hardened binder mass, the 28 dcompressive strength of which is 60-160 Mpa, preferably 80-120 Mpa. 11.The mix according to claim 9, wherein the mix can be hardened to form ahardened binder mass, the compressive strength of which is obtained fromthe graph in FIG. 3 according to the following formula${X = \frac{{2.06\mspace{14mu} v_{c}c\;\alpha} + V_{MKA}}{{v_{c}c\;\alpha} + V_{MKA} + W_{1}}},$wherein X=gel-space ratio v_(c)=specific volume of cement, dm³/kgc=weight of cement, kg α=degree of hydration, % V_(MKA)=volume ofmetakaolin agglomerates, dm³ W₁=volume of water, dm³=total watervolume−the water volume absorbed by agglomerates.
 12. A process forpreparing a hydraulically hardened binder mass, comprising: forming apaste-like composition from a hydraulically hardening binder, apozzolanically reacting admixture and water, which, when so desired,contains stone aggregate or similar filler; using spherical, porousagglomerates, which at least partially consist of metakaolin particles,as the pozzolanically reacting admixture; processing the paste-likecomposition; allowing the worked composition to harden to form thebinder mass; wherein the spherical, porous agglomerates having anindividual agglomerate size of 2-200 microns are used as thepozzolanically reacting admixture, whereby the density of the surfacepart of individual agglomerates is lower than that of the inner part andthe pore structure of individual agglomerates in the surface part andthe inner part is essentially the same; and the difference between theamount of water required by the workability of the paste formed from thebinder and the admixture and the smaller amount of water required by thehydration reactions is stored in a reversible way, after processing thebinder paste, in the pore structure of the said spherical, porousagglomerates and returned in subsequent hardening phases of the binderpaste.
 13. A process according to claim 12, wherein a binder mass isprepared, the portion of the spherical, porous agglomerates of the totalamount of the binder and the admixture being about 5-35% by weight. 14.A process according to claim 12, wherein agglomerates are used, the sizeof which is 5-200 microns and the average size d50) of which is the sameor essentially the same as the average size d50) of the particles of thecement-containing binder used in the mix.
 15. A process according toclaim 12, wherein the filling of the pores of the porous agglomeratesused in the mix is adjusted so that it begins within 1-60 minutes fromthe time, when the components of the binder mass have been mixedtogether.
 16. A process according to claim 12, wherein the porestructure of the porous agglomerates used in the mix is filled withwater within 20 minutes from the time the components of the binder masshave been mixed together, preferably so that 90% of the pore volume isfilled within 10 minutes from the termination of mixing.
 17. A processaccording to claim 12, wherein the filling speed of the porousagglomerates used in the mix is adjusted, preferably increased usingmechanical methods that are commonly used in connection withdampproofing cement, such as vibration or pressurization so that 90% ofthe pore volume is already filled within 20-60 seconds.
 18. The mixaccording to claim 1, wherein the mix can be hardened to form a bindermass, which has a porous matrix formed by strong agglomerates, in whichmatrix the gap between the pores is 20-100 microns.